In a lithography process for manufacturing a semiconductor element, liquid crystal display element, or the like, an exposure apparatus has been used. A reduction projection exposure apparatus (stepper), of a step-and-repeat method, which transfers a pattern formed on a mask or reticle (to be generically referred to as a “reticle” hereinafter) through a projection optical system onto a plurality of shot areas on a substrate such as a wafer or glass plate (to be referred to as a “wafer” hereinafter) in turn, and a moving-one-by-one type projection exposure apparatus (to be referred to as a “stepper” hereinafter as needed) such as a scanning projection exposure apparatus (scanning-stepper), of a step-and-scan method, which is obtained by improving the reduction projection exposure apparatus are mainly employed.
Important ones of the basic factors to determine the performance of the stepper are the resolution ability (resolution) and focus depth of the projection optical system (projection lens). It is because the resolution determines the finest pattern size that can be projected and imaged, i.e. the narrowest line width in practice (device rule). And its focus depth has to be as wide as possible because a processed wafer has steps formed thereon by forming and etching thin layers of oxide, metal, etc., and may be slightly deformed.
The resolution R and focus depth DOF of a projection lens is given by the following two equations as a general measure:R=k1·λ/N.A.  (1)DOF=k2·λ/(N.A.)2  (2)
Note that λ, N.A. and k1, k2 respectively represent the wavelength of light used for exposure, the numerical aperture of the projection optical system, and parameters determined by resist, etc.
An early stepper used a g-line (λ=436 nm) of an ultra-high pressure mercury lamp as exposure illumination light; its N.A. was about 0.3, and its resolution was about 1.2 um. As obvious in the above equation (1), the larger the N.A. is, the better the resolution is. With a larger N.A., however, the focus depth is decreased as obvious in the above equation (2).
Therefore, apparatuses for production of 16 Mbit DRAM and apparatuses for the later have achieved high resolution by using i-line (λ=365 nm) and a projection optical system of which N.A. is about 0.5, and also have achieved a practical focus depth. Because the i-line can be obtained from an ultra-high pressure mercury lamp like the g-line and can be used in the same way as earlier steppers in semiconductor manufacturing factories, the i-line steppers have been introduced as a production unit more smoothly than expected.
After that, in the period from the third generation of 64 Mbit DRAM to 245 Mbit DRAM where their device rules are equal to or less than 0.3 um, instead of the mercury lamp as a light source, the use of KrF excimer laser was examined, and the application of a phase shift reticle and modified illumination that can be applied by improving then current units was suggested.
While a KrF excimer laser (λ=248 nm) stepper uses a common basic body with a previous stepper using g-line or i-line, its optical system from the source to the projection optical system is different. That is, because using light of a shorter wavelength, usable optical materials are limited to quartz, fluorite, etc. In addition, because excimer laser light is pulse emission light and coherent unlike that of a mercury lamp, a special technology for the illumination system is necessary.
Because interference fringe induced by the coherency of the excimer laser affects the precision of line width of an exposure pattern, it is necessary to suppress the effects of the interference fringe by providing a vibrating mirror in the illumination optical system and making a light beam fluctuate finely. In a stepper with a lamp as its light source, accumulated exposure amount can be controlled by the open/close time of its shutter. Meanwhile, in a laser-light-source stepper using pulse emission, it is necessary to set the number of pulses per an exposure to be equal to or more than a certain number (referred to as a minimum exposure-pulse number) to compensate for energy fluctuation.
For 1 Gbit DRAM of the future where device rules will be equal to or less than 0.2 um, the use of ArF excimer laser (λ=193 nm), phase shift reticle and modified illumination technology is suggested. An ArF excimer laser stepper needs to have the optical path of its exposure light filled with nitrogen or the like because the exposure light is absorbed by oxygen.
To obtain even higher resolution and wider focus depth, light source of a shorter wavelength can be used leaving N.A. of the projection optical system as it is. Recently, reduction projection lithography using F2 (λ=157 nm) laser having a shorter wavelength than ArF excimer laser as its light source is presented by MIT Lincoln Lab., and is attracting a lot of attention. Because F2 laser light is absorbed by oxygen like ArF excimer laser light, exposure needs to be performed in the atmosphere of N2 or He.
Furthermore, to improve the precision of an exposure apparatus using light source of such a short wavelength, the precision of positioning of a reticle stage and wafer stage needs to be improved, and it is also necessary to suppress vibrations of units due to reaction caused by the drive of the stages as movable objects as much as possible and insulate floor vibrations.
Therefore, to realize highly precise exposure using such a short-wavelength light source as F2 laser, it is necessary to control positioning, and suppressing or insulating vibrations more precisely than ever.
For example, as factors required of a scanning stepper that transfers the reticle pattern onto a plurality of shot areas on the wafer in turn according to a step-and-scan method by repeating a scanning exposure operation, in which a reticle pattern is transferred onto the wafer synchronously moving the reticle stage holding the reticle and the wafer stage holding the wafer in one dimensional direction, and a stepping-between-shots operation of the wafer stage, there are the following things: [1] The driving reactions of the stages do not transmit to the frame supporting the projection optical system; [2] It is easy to adjust relative position and attitude between each of the stages and the projection optical system; [3] The driving thrust and driving reaction are reduced by making the stages lightweight; [4] It is so structured that floor vibrations are prevented from transmitting to the stages and the projection optical system; [5] It is so structured that the stages and the projection optical system can be easily separated. [1] to [4] are required to realize precise exposure of a fine pattern, and [5] is required to improve easiness of its maintenance and reduce its down-time so that the productivity of a micro device is improved.
Also, there is a double wafer-stage scheme for realizing the high throughput where two wafer stages are mounted on the level block; while a wafer stage is performing exposure, the other performs alignment or the like and the two operations are alternately and continuously performed. Furthermore, a multi-exposure method such as a double exposure method where using two reticle stages each holding a reticle having a pattern different from the other formed thereon, those patterns are transferred in turn is effective for higher integration of semiconductor devices because it is possible to improve the resolution and focus depth. Note that to achieve a desirable effect in the double wafer-stage scheme and the multi-exposure method, the above factors [1] to [4] have to be satisfied as a premise, and that it is preferable to satisfy the above factor [5].
In addition, when performing exposure using light of a wave length shorter than or equal to F2 laser light, [6] it is required that the stages, the projection optical system, and the like should be contained by a chamber so as to be enclosed in the atmosphere of N2 or He.
Also, in a step-and-repeat type stepper, it is preferable to satisfy the above factors [1] to [6], not to mention.
Of the above factors [1] to [6], the factor [3] can be satisfied by adopting a planar motor or a cylinder-like linear motor as driving sources of the stages. Especially, when adopting the planar motor as a driving source of the wafer stage, the planar motor can drive the movable object in three degrees of freedom X, Y, θz by one motor, and therefore, X guide and Y guide of X-Y two dimensional arrangement that were previously indispensable are unnecessary. And when driving the stage holding the wafer in one direction of X-axis and Y-axis directions, a motor and a guide for driving the stage in the other direction need not be driven along with the stage, and therefore, the weight of the movable portion can be remarkably reduced. Therefore, it is attracting a lot of attention as a promising driving source of the wafer stage of an exposure apparatus in the future.
However, it is very difficult to satisfy the rest, [1], [2], [4], [5], [6], at the same time even if the planar motor is adopted as a driving source of the stage.
Meanwhile, as means of moving the movable portion in the six degrees of freedom regarding translation and rotation, a mechanism referred to as a steward-platform, a kind of parallel mechanism (also referred to as a parallel link mechanism) where a base portion and end-effecter are connected by a plurality of link chains each having more than two joints, is used in training and simulation of flying an air plane, and is attracting attention in the field of industrial robots.
The present invention is invented under such a circumstance and a first purpose of the present invention is to provide a parallel link mechanism that can realize the miniaturization/lightening and the improvement of the output at the same time.
Furthermore, a second purpose of the present invention is to provide an exposure apparatus that can precisely transfer a fine pattern onto a substrate and a method of making the exposure apparatus.
In addition, a third purpose of the present invention is to provide a method of manufacturing a highly integrated micro-device with high productivity.